The hefty fine comes after the government agency investigated how the airline handled delays at the Denver airport during a snowstorm in December 2016. Between December 16 and 18, the Denver-based airline forced 12 flights to wait on the tarmac for more than three hours, without allowing passengers to deboard the plane. Department regulations state that “U.S. airlines with 30 or more passenger seats are prohibited from allowing their domestic flights to remain on the tarmac for more than three hours without giving passengers an opportunity to leave the plane,” which is exactly what Frontier did.

According to passengers who spoke to Denver’s local KDVR, the entire cabin on one of the flights was chanting “let us out of here” for hours. “I absolutely felt like we were held hostage,” said one flier. Frontier, for its part, blamed the debacle on “operational challenges,” even though the airline was fully staffed and had called in extra help from its nearby headquarters.

This isn't the first time Frontier has been scrutinized for its policies. In May 2016, four female pilots filed a complaint saying that they were prevented from breastfeeding at work.

The airline won’t have to pay the whole $1.5 million though: $900,000 will be credited back to account for the compensation it already paid to passengers on the affected flights and on others delayed during the storm. That said, the fine is the second highest amount an airline has received for violating tarmac delay regulations. American and Southwest are tied for first, after both airlines were fined $1.6 million for keeping passengers on the tarmac for more than three hours. American was fined for violations that occurred on 27 flights, while Southwest was penalized for 16 flights.